High precision of press worked products (precision of shapes and dimensions is high), and speedup of pressing work to improve productivity have been long demanded in recent years. As a press to meet this demand, the one, in which a ball screw is linearly driven in an up-and-down direction with, for example, a servo motor and thereby the position and speed of the slide are controlled with high precision to drive the slide precisely up and down, is proposed.
Meanwhile, the one, in which die height (so-called height from a top face of a bolster to a bottom face of a slide at the bottom dead center of the slide) is adjusted with a motor, is proposed in order to attain high precision of press worked products and speedup of press production as described above. For example, Japanese Utility Model No. 61-24392 discloses a slide adjusting device shown in FIG. 8, and it will be explained below based on FIG. 8. A frame 40 is provided with a plunger 52 to be movable up and down, the plunger 52 is connected to a connecting rod 51 with a pin 41. A small diameter part 42 is formed at a lower part of the plunger 52, and a step part 43 is formed between the small diameter part 42 and an upper part of the plunger 52. A screw hole 44 is formed at the small diameter part 42 from a lower surface toward an upper position, a slide adjusting screw 53 is fitted in the screw hole 44, and a gear 54 is formed at a lower part of the slide adjusting screw 53.
The lower part of the plunger 52 is fitted in a hole-shaped part 46 of a slide 45 via an O-ring. The gear 54 of the slide adjusting screw 53 abuts to an upper surface 46a of a gear box of the slide 45, and a clearance is provided between a lower surface of the gear box and the gear 54. An oil chamber C is formed between the plunger 52, the step part 43, and a step part 47 of the hole-shaped part 46 of the slide 45. A slide adjusting motor 48 is attached to the slide 45, and a pinion 49 of the slide adjusting motor 48 is meshed with the gear 54 via an intermediate gear 50. The oil chamber C communicates with an output port of a change-over valve 55, and a pump port of the change-over valve 55 is connected to a discharge side of a pump 57. A return port of the change-over valve 55 communicates with a tank 58 via a check valve 59 which is opened at set pressure. The output port of the change-over valve 55 communicates with the tank 58 via an over load valve 60.
However, the following problems occur to the above-described conventional die height adjusting device.
(1) When an ordinary induction motor (induction motor) is used as a die height adjustment driving source, a predetermined contactor (electro-magnetic switch) is operated to be on for a predetermined period of time by an inching operation by a button operation of an operator, or the like to drive the induction motor at a predetermined voltage at a time of die height adjustment. As a result, a slide adjustment amount is varied, and adjustment cannot be performed at high precision in a unit of 1/100 mm. The demand for higher precision (within a few microns) cannot be met by any means.
(2) A high-speed operation of a pressing machine is demanded to enhance productivity, but heat generated by the high-speed operation causes thermal expansion of a frame and the like, and gradually changes die height. Such a change in die height is dealt with by adjusting the die height, but the die height cannot be adjusted without stopping the pressing machine or stopping the slide at the top dead center.
(3) A high-speed operation of the pressing machine causes very large impacts and vibrations (for example, the maximum acceleration 50 G, G=9.8 m/s2) to the slide 45 at the time of forming and after forming. Consequently, use of a servo motor as the slide adjusting motor 48 attached to the slide 45 to perform highly precise die height adjustment causes a problem in durability.